The wireless communications industry has grown immensely in the past twenty years. In particular, the use of cellular or wireless phones and other such devices has been widely accepted by the public. The first generation of such cellular telephone devices operated on an analog transmission principle using a pair of frequencies per transmission session (sending and receiving) per user. Shortly after cellular devices became popular, digital cellular devices were perfected and appeared in the marketplace. Digital cellular devices operate by modulating a digital signal onto a carrier wave. Since the signal is digital, certain techniques can then be used to allow multiple users access to the same frequency spectrum, thereby effectively increasing system capacity.
One such technique, called Code Division Multiple Access (CDMA), provides a multiple access technique that offers certain benefits over analog cellular transmission techniques. In CDMA, a single radio frequency is used as a carrier for the communications of multiple users. By way of example, two CDMA wireless telephone users who are each making a call may have communications simultaneously transmitted using one or more of the same radio frequencies. Each user's signal is encoded with a pseudorandom noise (PN) code at the transmitter in such a way that it can be properly decoded at a received with minimal interference to another user's signal.
Certain types of interference problems however still exist in CDMA systems. For instance, a fading characteristic called Rayleigh fading occurs when a transmission signal is reflected from many different features of the physical transmission environment. As a result, the signal arrives at the destination receiver from many directions and each signal may have a different transmission delay. These effects can result in destructive summation of the signals, otherwise known as multi-path fading.
Certain CDMA modulation techniques can be used to mitigate the adverse effects of multipath fading and interference, while exploiting frequency reuse advantages. One such prior art modulation technique widely implemented in CDMA systems uses orthogonal codes. Using this technique, a digital data signal to be transmitted is first modulated with a pseudorandom noise (PN) code. The resulting signal is then modulated with an orthogonal code created from an orthogonal code generator to create a user signal which is orthogonal to other user signals. The orthogonal signal can be transmitted on the same frequency as an orthogonal signal created with an another code that is orthogonally related to the first orthogonal code. That is, both orthogonal codes are mathematically interrelated such that the signals produced from the modulations using each code are non-interfering when transmitted on the same frequency together.
Benefits of digital CDMA systems include a decreased requirement for transmission power which results in longer battery life, increased clarity, higher radio frequency reuse, and favorable signal-to-noise ratio results. Since the frequency spectrum can be reused multiple times in such systems, an overall increase in system user capacity also results. For specific details of the use of CDMA techniques in a multiple access system, the reader is referred to U.S. Pat. No. 4,901,307.